Some of them just weren't ready to have kids. If i were a rock/rap star, I wouldn't have children until I was certain that I no longer wanted to throw lavish sex parties at my pad, drive my rolls royce into my empty swimming pool, kick random photographers in the groin, or have sex with underaged groupies. Only then would I consider having kids of my own.
Basically, I wouldn't have children until I was impotent.